1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and, more particularly, to a display device that has a low driving voltage, superior life span characteristics, and is stabilized to the external environment due to an improved electron transport layer and electron injection layer.
2. Description of Related Art
Recently, the organic electroluminescent display device is being noticed as a next generation display device owing to various qualities such as its narrow thickness, wide viewing angle, light weight, small size, fast response time, and low power consumption compared with a cathode-ray tube (CRT) or liquid crystal device (LCD).
Particularly, an organic electroluminescent display device has a quality of being easily fabricated through a simple fabrication process, due to its simple structure of an anode, organic film layer, and a cathode. The organic film layer consists of various layers according to functions, and may generally comprise a hole injection layer, a hole transport layer, a luminescent layer, an electron transport layer, and an electron injection layer.
In FIG. 1, holes are injected from the anode 7, which is a transparent electrode, so that the injected holes are transferred to an emitting layer 4 which is one of organic film layers 10 through a hole injection layer 6 and a hole transport layer 5, and electrons are injected from a cathode 1 so that the injected electrons are transferred to the emitting layer 4 through an electron injection layer 2 and an electron transport layer 3. The transferred electrons and holes are bonded in the emitting layer 4 to emit light.
There are methods for improving an electron transport layer and an electron injection layer in order to improve the driving voltage and life span of an organic electroluminescent display device. According to U.S. Pat. No. 5,776,622, characteristics of a device were improved by depositing a dielectric material such as LiF, BaF2 and CsF to a thickness of about 1 nm between an electron transport layer and a metallic electrode to improve driving voltage.
Kido, et al., of Japan announced results in which a driving voltage was reduced by using a layer in which a material in the alkaline metal series, such as Li, is doped on an organic electron transport layer adjacent to the cathode as an electron injection layer (SID 97, Digest, page 775).
The reduction of the driving voltage by the doping of an alkaline metal results from the fact that electrons are more smoothly injected by removing an electron injection barrier from the cathode. However, the material in the Li series has a problem in that it is sensitive to oxidization, and is diffused to a large extent. Furthermore, a finely controlled doping concentration is required, since the concentration of a doping metal has an influence on the characteristics of a device in the case of metal doping.
Disclosed in U.S. Pat. No. 6,396,209 is a structure in which a mixed layer of an electron transporting organic compound and an organic metal complex compound is used as an electron injection layer, and a metal is deposited on the mixed layer to improve the characteristics of an organic electroluminescent display device. Using this electron transporting organic compound and the organic metal complex compound could reduce the driving voltage.
However, a structure of a new device for obtaining more stable device characteristics is required, since the methods discussed above have a high degree of difficulty in the depositing process, and also have a low device stability.